Protesters In London Fly Giant Orange Baby Trump

'Baby Trump' sure is a sight to see.

Opponents of Donald Trump have flown a six-meter blimp depicting the US president as an orange, snarling nappy-wearing baby just outside the British parliament.

Trump, who arrived in Britain on Thursday, told the Sun newspaper planned protests against him in London and other British cities made him feel unwelcome so he was avoiding the capital as much as possible.

"I guess when they put out blimps to make me feel unwelcome, no reason for me to go to London," Trump told the newspaper.

"I used to love London as a city. I haven't been there in a long time. But when they make you feel unwelcome, why would I stay there?"

The 'Baby Trump' balloon was inflated in London's Parliament Square. Image: AAP

Britain regards its close ties with the United States, which it calls the special relationship, as a pillar of its foreign policy and Prime Minister Theresa May has courted Trump ahead of the country's departure from the European Union.

But some Britons see Trump as crude, volatile, unreliable and opposed to their values on a range of issues. More than 64,000 people have signed up to demonstrate in London against his visit while other protests are expected around the country.

A few hundred people gathered to watch the blimp launch in Parliament Square, with organisers of the stunt wearing red boiler suits and red baseball caps emblazoned with "TRUMP BABYSITTER".

US Marine helicopters pass the 'Baby Trump' balloon as it rises after being inflated in London's Parliament Square. Image: AAP

After counting down from 10 to 1 a cheer went up as the large balloon rose to fly around 10 metres off the ground, next to parliament and the River Thames.

Organiser Daniel Jones, a charity communications officer aged 26, said they were trying to make people laugh as well as making a serious point.

"It's also about giving a boost to those in America resisting his policies," he said.

London Mayor Sadiq Khan, who was criticised by Trump in the Sun interview for failing to control crime and prevent militant attacks, gave his blessing for the blimp to be flown and rejected suggestions this showed a lack of respect to the US president.